This invention relates to a variable reflectivity filter, and more particularly to an etalon having one or more end faces having a variable reflectance.
Fabry-Perxc3x4t filters are widely used in optical systems where a dependence upon their periodic nature is required. For example, comb filters are now being developed wherein one or more etalons provide one or more output signals that are periodic in amplitude response. The free-spectral range, or period of a Fabry-Perot etalon is determined by a length of a gap between its two reflecting surfaces.
A multi-port tunable fiber-optic etalon filter (MTFET) having two spaced partially reflective mirrors is disclosed in U.S. Pat. No. 5,283,845 in the name of Ip assigned to JDS Fitel Inc. This filter has three or more ports, with at least two on one side and at least one on the other side of the etalon. A single signal can be filtered with the reflected signal being received, forming a wavelength division multiplexor, or a plurality of signals can be filtered, with or without the reflected signals being received. Ip provides a filter wherein the FSR is tunable within a range.
Another use for an etalon is described by Ip in U.S. Pat. No. 5,557,468 in the name of Ip assigned to JDS Fitel Inc., wherein an etalon is used a dispersion compensation device.
Yet another use for an etalon is described in U.S. Pat. No. 5,798,859 in the name of Colbourne et al. assigned to JDS Fitel Inc. and device for wavelength locking is provided, wherein an element having a wavelength dependent characteristic such as a Fabry-Perxc3x4t etalon is used to provide an output signal having an intensity that varies with wavelength. The intensity of a reference signal derived from an input signal is compared with an output from the Fabry-Perxc3x4t etalon to provide a feedback signal that corresponds to the frequency of the input signal. The system is calibrated before wavelength locking is performed to determine a ratio of intensities that determines a locked state or condition.
As can be seen from the above mentioned patents, Fabry-Perxc3x4t etalons have a variety of uses and are provided in a variety of forms.
Another type of etalon that differs from a fixed etalon, or even a tunable etalon as is described by Ip in U.S. Pat. No. 5,283,845 is one wherein the finesse or reflectivity of one or more of the reflectors is variable. Such a filter is described in U.S. Pat. No. 5,452,121 in the name of Hilgeman assigned to Northrop Grumman Corporation and is said to be useful as a spectral resolution agile filter.
The instant invention provides an etalon having a variable finesse over the length of at least one reflector.
One particular use for a filter of this type, is in the field of optical amplifiers and more particularly for gain tilt control.
Optical amplifiers and particularly erbium doped optical fiber amplifiers are nearly ubiquitous in optical transmission systems, particularly in the field of telecommunications. Erbium doped fiber amplifiers (EDFAs) have high polarization insensitive gain, low cross talk between signals of different wavelengths, good saturation output power, and a noise figure close to the fundamental quantum limit. The excellent noise characteristics allow hundreds of these amplifiers to be cascaded to cover spans of thousands of kilometers of optical fibre. EDFAs as opposed to electronic repeaters are also transparent to data rate, signal format, and wavelength over a limited range, making them useful for wavelength multiplexed (WDM) communication systems that simultaneously transmit a large number of signals using different wavelength bands for each signal.
Notwithstanding these generally excellent characteristics, a disadvantage associated with EDFAs is their narrow spectral width and uneven gain band. The useful telecommunications window of an EDFA is approximately 20-30 nm wide, while an ideal amplifier would have a flat spectral gain across the full spectrum which extends form approximately 1520 nm to 1570 nm. The peak wavelength of the erbium gain spectrum varies from about 1530 nm to about 1535 nm depending upon the host glass material. FIG. 1 shows the characteristic gain spectrum of a particular conventional EDFA where it can be seen that the gain as a function of wavelength varies; this variation will be referred to hereinafter as gain ripple. Numerous techniques have been published for widening and flattening the gain spectrum (i.e. reducing the ripple) and include for example co-doping an erbium-doped silica glass fibre with Al2O3; changing the host glass material itself; using various forms of attenuating filters to reduce the gain at the emission peak; and, constructing hybrid devices having two or more different types of serially connected erbium doped fibre and actively adjusting pump conditions independently in each fibre section to compensate for the different gain slopes of each fibre.
In addition to the aforementioned problems and solutions associated with minimizing gain ripple, another significant problem exists to which there have been no simple, inexpensive, and practicable solutions. This other significant problem solved by this invention relates to improving dynamic gain tilt. The term dynamic gain tilt as used hereafter means the variation in gain at one wavelength as a result of changing the gain at any other wavelength via a change in input EDFA operating conditions. Although the techniques described above for minimizing gain ripple can provide a relatively flat spectrum in a specified wavelength band for a specific set of input optical powers and wavelength, the gain equalization performance degrades rapidly when the gain is changed (change in average population inversion levels) from the nominal conditions by changing the input power to the amplifier. One reported solution to this problem is allegedly achieved by a hybrid fibre device having cascaded amplifying stages with different gain spectra and an equal number of pump sources to allow the gain spectra of the individual stages to be effectively tuned independently so that when the total gain is changed, the relative contribution of each stage can be adjusted to arrive at the desired gain, with a resulting gain spectrum having a minimal amount of spectral distortion over the selected wavelength band. As an example, an erbium doped fibre having a positive gain slope may be combined with a different erbium doped fibre having a negative gain slope such that the hybrid device has a nearly flat gain at specific input power conditions. However, if the overall gain of the hybrid device must be changed, the gain slope of each of the constituent states will generally change at different rates when the pump power input to one of the stages is changed. In order to achieve good compensation at the new operating point, the relative gain of each of the constituent gain stages must be readjusted to make the gain slopes compensate each other. In implementing this type of amplifier, one skilled in the art would likely cascade two or more different erbium doped fibre compositions and provide a separate pump source for each amplifying stage at an end of each stage so as to minimize the number of splices and make it as convenient as possible to independently control the pump power to each stage. However, this technique for reducing or improving dynamic gain tilt requires a complex control scheme during operation in which the total power of multiple pump sources must be coordinated in order to realize gain slope compensation over a range of different gains (i.e. to change input power while maintaining a fixed target output power).
U.S. Pat. No. 5,764,406 in the name of Newhouse et al. entitled Hybrid Optical Amplifier Dynamic Gain Tilt incorporated herein by reference, describes a system wherein an erbium doped fibre amplifier device has a dynamic gain tilt that is less then the gain tilt of any of the constituent fibres. The hybrid device has at most one less pumping source than the number of constituent waveguides of the device. The hybrid device automatically provides a change in the pump distribution among the constituent doped waveguide sections so as to achieve a readjustment of the relative gains of the constituent sections. In one embodiment, this invention provides constituent EDFs of different co-dopant compositions that provides an automatic change in the pump distribution or partitioning among the constituent EDF sections so as to achieve a readjustment of the relative gains of the constituent EDF sections.
Although the ""406 patent appears to achieve its intended function, it is a relatively costly and is a complicated solution to dynamically controlling gain tilt.
Most known solutions for correcting for dynamic gain tilt have an associated power loss (approximately 5 dB) and furthermore, increased power is required for extra pumping. Another deleterious result of these systems is an increased resulting noise.
It is an object of this invention to provide a novel filter and method of use of such filter that can be placed in-line within an optical amplifier for varying the gain tilt dynamically as the power of the input signal is varied.
It is another object of the invention to provide an inexpensive filter for use in controlling tilt gain of an optical amplifier.
It is yet another object of the invention to provide an inexpensive etalon filter having a variable reflectance on one or more end faces suitable for use, for example in controlling tilt gain of an optical amplifier, but not limited thereto.
In accordance with the invention there is provided, a Fabry-Perxc3x4t etalon having two at least partially reflective end faces, at least one of the end faces having a variable reflectance along its length. The variable reflectance may be variable in a continuous linear manner over a particular region, or may be step-wise varied over at least two regions.
In accordance with the invention there is provided, a resonant optical cavity having a free spectral range greater than 20 nm, said cavity having a first end face and an at least second partially reflective end face, the first end face being partially reflective and having a reflectivity along its length which varies by at least 10%.
In accordance with the invention, there is provided, a variable slope optical filter for in-line use with an optical amplifier signal, the filter being disposed at an input side, an output side, or within the optical amplifier, the filter having a wavelength response that is substantially linear in slope within a band of operation wavelengths of the amplifier, the slope of the filter in said band of wavelengths being between zero and a only one of a positive or negative number, the filter for passing a band of wavelengths having a centre wavelength xcexc and having an amplitude response that has an opposite and counter slope as a function of wavelength to that of the amplifier""s gain tilt within the operation band of wavelengths; and, a means for providing relative movement between the filter and the optical amplifier signal to vary the slope of the filter within the band of operation wavelengths. It should be noted, that relative movement may be actual displacement of one of the filter and the input port or, alternatively, by varying the relative angle between the input port and the filter.
In accordance with the invention there is further provided, an optical amplifier having coupled thereto, an optical filter for dynamic and variable gain tilt control for use with the amplifier, the filter in a predetermined wavelength band having a width of at least 10 nm where gain tilt control is required, having a centre wavelength xcexc and an amplitude response that has an opposite and counter slope as a function of wavelength to that of the amplifier within the predetermined wavelength band; and, a controller and movement mechanism for varying the slope of the filter in response to a control within the predetermined band, the output response of the filter having a slope throughout the predetermined wavelength band which is always only one of negative and positive.